The Pentagon announced Tuesday that 15 prisoners from Guantánamo Bay have been released into custody of the United Arab Emirates, the largest transfer during President Obama's administration and a milestone in his effort to close the high-security, offshore prison.

The release of 12 Yemenis and 3 Afghans brings the number in detainment at the prison in Cuba to 61, the BBC reported, but efforts to close the prison entirely demand congressional approval and remain no less controversial than the prison itself.

This represents a major success for Mr. Obama, for whom closing the prison was a campaign promise. He believes that the facility where suspected terrorists have been held for years without trial and were previously interrogated via waterboarding, harms anti-terror work in other countries.

"The continued operation of the detention facility weakens our national security by draining resources, damaging our relationships with key allies and partners, and emboldening violent extremists," said Lee Wolosky, the State Department's special envoy for closing the Guantánamo detention center, according to Reuters. "The support of our friends and allies – like the UAE – is critical to our achieving this shared goal."

It is also the second time the UAE has resettled detainees from the prison. The first transfer appears to have gone smoothly, suggesting the country might prove willing to take more.

"From what we've learned, they've been treated pretty well," Clive Stafford Smith, the director of the British advocacy group that represented a Yemeni prisoner released previously told The Associated Press. "They've been banned from traveling and any meaningful communication.... They've actually been OK. Arabic is the main language and its pretty close to home."

Republicans and some Democrats in the United States, however, say the international outreach is not worth the risk to US security that comes from releasing the type of prisoners who have been held at the naval base.

"In its race to close Gitmo, the Obama administration is doubling down on policies that put American lives at risk," Rep. Ed Royce (R) of California, chairman of the House Foreign Affairs Committee, said in a statement. "Once again, hardened terrorists are being released to foreign countries where they will be a threat."

The alternative to releasing detainees to foreign nations is transferring them to US prisons, a move that Congress has steadfastly opposed, saying that they do not belong with American civilian prisoners. The debate is further complicated by former Guantánamo prisoners who have actually rejoined terrorist groups after their release, although the record appears to be improving: At least 21 percent of detainees released during the Bush administration re-engaged in terrorism, compared to 5 percent during Obama's, according to US intelligence officials.

As a result of this concern and in an effort to act more decisively on terror, Republican nominee Donald Trump has said he wants to keep the base's prison capacity open, fill it with "bad dudes" and "bring back ... a lot worse than waterboarding."

The move to close the prison is welcome to human rights groups that say the prison weakens US efforts to discourage torture and unlawful detention in other countries. Amnesty International has repeatedly urged the president to work toward closing the prison before he leaves office,

"I think we are at an extremely dangerous point where there is a significant possibility this is going to remain open as a permanent offshore prison to hold people, practically until they die," Naureen Shah, security and human rights program director for Amnesty International US told Reuters.